In the stage of data analysis is a study of the phase-resetting mechanism of the hypothesized internal clock. It is hoped that the results will narrow the divergencies between the traditionally opposed endogenous and exogenous points of view. In the final stages of data gathering is an investigation of changes in the rat's activity pattern as a function of aging. In the early stages of data gathering is an experiment in which longevity in rats is examined as a function of weekly reversal of light/dark illumination regimen (and presumably repeated episodes of internal desynchronization). It is already demonstrated that such reversals adversely affect the rate of development. There will be an extension of some preliminary observations that the availability of an exercise wheel in old age increases longevity. It is hoped that studies on the effect of early exposure to unusual patterns of ambient illumination on free-running activity pattern, susceptibility to induced ulceration, and susceptibility to audiogenic convulsion can be carried out.